Digital photography has been growing fast for several years and the general public now has access to efficient and reasonably-priced digital cameras. Therefore people are seeking to be able to produce photographic prints from a simple computer and its printer, with the best possible quality.
Many printers, especially those linked to personal office automation, use the inkjet printing technique. There are two major families of inkjet printing techniques: continuous jet and drop-on-demand.
Continuous jet is the simpler system. Pressurized ink (3.105 Pa) is forced through one or more nozzles so that the ink is transformed into a flow of droplets. In order to obtain the most regular sizes and spacing between drops, regular pressure pulses are sent using, for example, a piezoelectric crystal in contact with the ink with high frequency (up to 1 MHz) alternating current (AC) power supply. So that a message can be printed using a single nozzle, every drop must be individually controlled and directed. Electrostatic energy is used for this purpose: an electrode is placed around the ink jet at the place where drops form. The jet is charged by induction and every drop henceforth carries a charge whose value depends on the applied voltage. The drops then pass between two deflecting plates charged with the opposite sign and then follow a given direction, the amplitude of the movement being proportional to the charge carried by each of the plates. To prevent other drops from reaching the paper, they are left uncharged: so, instead of going to the support they continue their path without being deflected and go directly into a container. The ink is then filtered and can be reused.
The other category of inkjet printer is drop-on-demand (DOD). This constitutes the basis of inkjet printers used in office automation. With this method, the pressure in the ink cartridge is not maintained constant but is applied when a character has to be formed. In one widely used system, there is a row of twelve open nozzles, each of them being activated with a piezoelectric crystal. The ink contained in the head is given a pulse: the piezo element contracts with an electric voltage, which causes a decrease of volume, leading to the expulsion of the drop by the nozzle. When the element resumes its initial shape, it pumps the ink necessary for new printings into the reservoir. The row of nozzles is thus used to generate a column matrix, so that no deflection of the drop is necessary. One variation of this system replaces the piezoelectric crystals by small heating elements behind each nozzle. The drops are ejected following the forming of bubbles of solvent vapor. The volume increase enables the expulsion of the drop. Finally, there is a pulsed inkjet system in which the ink is solid at ambient temperature. The print head thus has to be heated so that the ink liquefies and can print. This enables rapid drying on a wider range of products than conventional systems.
New “inkjet” printers capable of producing photographic images of excellent quality are now available. However, they cannot supply good proofs if inferior quality printing paper is used. The choice of printing paper is fundamental for the quality of the resulting image. The printing paper must combine the following properties: high-quality printed image, rapid drying during printing, good image colorfastness over time, and smooth and glossy appearance.
In general, the printing paper comprises a support coated with one or more layers according to the properties required. It is possible, for example, to apply on a support an etch primer layer, an absorbent layer, an ink dye fixing layer and a protective layer or surface layer to provide the glossiness of the material. The absorbent layer absorbs the liquid part of the water-based ink composition after creation of the image. Elimination of the liquid reduces the risk of ink migration to the surface. The ink dye fixing layer prevents any dye loss into the fibers of the paper base, to obtain good color saturation while preventing excess ink that would encourage the increase in size of the printing dots and therefore reduce image quality. The absorbent layer and fixing layer can also constitute a single ink-receiving layer accomplishing both functions. The protective layer is designed to ensure protection against fingerprints and the pressure marks of the printer feed rollers.
The ink-receiving layer usually comprises a binder, a receiving agent and various additives. The purpose of the receiving agent is to fix the dyes in the printing paper. The best-known inorganic receivers are colloidal silica or boehmite. For example, the European Patent Applications EP-A-976,571 and EP-A-1,162,076 describe materials for inkjet printing in which the ink-receiving layer contains as inorganic receivers Ludox™ CL (colloidal silica) marketed by Grace Corporation or Dispal™ (colloidal boehmite) marketed by Sasol. However, printing paper comprising an ink-receiving layer containing such inorganic receivers can have poor image stability in time, which is demonstrated by a loss of color density.
Furthermore, polyvinyl alcohol is generally used as binder. As this binder does not ensure the adhesion of the ink-receiving layer to the support, the combination of poly(alcohol) with hardeners, such as DHD (dihydroxydioxane) or sodium tetraborate (borax) is well known, especially in U.S. Pat. No. 6,419,987. The disadvantage of this combination is that it causes crackle or wavelet phenomena during the drying of the composition intended to form the ink-receiving layer. These phenomena can visibly alter the final quality of the printed image. The use of polyvinyl alcohol thus requires specific coating conditions that do not enable either cost reductions or productivity increases. Furthermore, hardeners can lead to unwanted reactions that result in a residual tint of the ink-receiving layer. Hardeners also tend to migrate, which can cause crosslinking in the surface of the ink-receiving layer, thus obstructing ink absorption.